Influence of pH and Surface Chemistry on Poly(L-lysine) Adsorption onto Solid Supports Investigated by Quartz Crystal Microbalance with Dissipation Monitoring

Poly(L-lysine) (PLL) adsorption onto various materials has been widely applied as a surface modification strategy and layer-by-layer fabrication method. Considering the role of electrostatic charges, a detailed understanding of the influence of solution pH on PLL adsorption process is important for optimization of PLL coating protocols. Herein, PLL adsorption onto different polar and hydrophilic substrates-silica, an amine-terminated self-assembled monolayer (SAM) on gold, and a carboxyl-terminated SAM on gold-across a range of pH conditions was investigated using the quartz crystal microbalance with dissipation. The adsorption kinetics consisted of an initial rapid phase, followed by a second phase where adsorption rate gradually decelerated. These features were interpreted by applying a mean-field kinetic model implying diffusion-limited adsorption in the first phase and reconfiguration of adsorbed PLL molecules in the second phase. The adsorption kinetics and uptake were found to be sensitive to the pH condition, surface chemistry, and flow rate. The strongest PLL adsorption occurred at pH 11 on all three surfaces while weak PLL adsorption generally occurred under acidic conditions. The surface morphology and roughness of adsorbed PLL layers were investigated using atomic force microscopy, and strong PLL adsorption is found to produce a uniform and smooth adlayer while weak adsorption formed a nonuniform and rough adlayer.